1. Field of the Invention
A method for supporting backward compatibility of Multimedia Broadcast and Multicast Service (hereinafter referred to as MBMS) in the mobile communication system of Wideband Code Division Multiple Access (hereinafter referred to as WCDMA) is proposed in the present invention. In order to guarantee the backward compatibility of MBMS and the compatibility among the equipments with different versions, the invention provides the method for obtaining the corresponding relationship between MBMS service identifier and Radio Access Bearer (hereinafter referred to as RAB) identifier by a Service GPRS (General Packet Radio Service) Support Node (hereinafter referred to as SGSN) and a User Equipment (hereinafter referred to as UE) when providing the MBMS service via a Radio Network Controller (hereinafter referred to as RNC) that does not support the MBMS.
2. Description of the Related Art
Multimedia Broadcast and Multicast Service (hereinafter referred to as MBMS) is a new service under standardization by 3rd Generation Partnership Project (hereinafter referred to as 3GPP). The MBMS service is a unidirectional point-to-multipoint (p-t-m) service, whose most remarkable characteristic is that it can make use of radio resources and network resources efficiently.
FIG. 1 shows the MBMS system structure. The MBMS network structure, based on the core network of General Packet Radio Service (hereinafter referred to as GPRS), has been added with new network units. Following is the description on the MSMS system structure in FIG. 1.
Broadcast and Multicast Service Center 101 (hereinafter referred to as BM-SC) is a service control center of the MBMS system. Gateway GPRS Supporting Node 102 (hereinafter referred to as GGSN) and Service GPRS Supporting Node 103 (hereinafter referred to as SGSN) constitute the transmission network of the MBMS service and provide route for data transmission. UMTS Terrestrial Radio Access Network 104 (hereinafter referred to as UTRAN) provides radio resources for the MBMS service over the air-interface. User Equipment 105 (hereinafter referred to as UE) is the terminal device for data receiving. Home Location Register 106 (hereinafter referred to as HLR) saves the data related to users and can provide services like user authentication. Uu107 is the radio interface, and Iu108 represents the interface between the access network and the core network. Radio resources used by the MBMS service are not dedicated for one user, but for all users using this service.
For the interface Iu, all users that joins in the same MBMS service share the same user interface. That is to say, one Radio Network Controller (hereinafter referred to as RNC) has only one MBMS data stream for the same MBMS service. In the RNC and the SGSN, the relationship of one-to-one correspondence lies between the user interface and the MBMS service identifier and MBMS Bearer Context.
In the system of Re199, Re14 and Re15, the Network Service Access Point Identifier (hereinafter referred to as NSAPI) is one-to-one correspondent to the Radio Access Bearer (hereinafter referred to as RAB) identifier, and in the Packet Service domain (hereinafter referred to as PS domain), it is also one-to-one correspondent to the Radio Bearer (hereinafter referred to as RB) identifier. When the UE initiates the process of activating the Packet Data Protocol (hereinafter referred to as PDP) context, it selects an unused NSAPI and transfers it to the SGSN. When the SGSN initiates the process of RAB allocating, it contains the NSAPI into the information element of RAB identifier.
The process that an existing UE joins in the MBMS service is illustrated in FIG. 2.
201 the UE initiates the process of setting up the PDP Context. Having been established successfully, the PDP Context is saved in the UE, the SGSN and the GGSN, and a signaling connection in PS domain is established between the UE and the GGSN. The intermediate devices for the signaling connection are Radio Access Network (hereinafter referred to as RAN) and the SGSN.
202 the UE transmits an Internet Multicast Manage Protocol (hereinafter referred to as IGMP) joining in message to the GGSN through the signaling connection established in step 201. The message contains the parameter of Internet Protocol (hereinafter referred to as IP) multicast address, which can identify a certain MBMS multicast service or a certain multicast service in external data network.
203 the GGSN and the BM-SC perform the signaling interaction to authenticate the UE.
204 the GGSN sends an “MBMS Notification Request” message to the SGSN, which contains parameters of UE identifier and IP multicast address.
205 after receiving the message in step 204, the SGSN sends a “MBMS Context Activation Request” message to the UE, which contains parameters of UE identifier and IP multicast address.
206 after receiving the message in step 205, the UE sends an “Activate MBMS Context Request” message to the SGSN, which contains IP multicast address and Access Point Name (hereinafter referred to as APN).
207 the SGSN sends a “MBMS Notification Response” message to the GGSN, which contains the value of reason. The value of reason indicates whether the MBMS context is activated successfully or not for the reason of the SGSN or the UE. When receiving the response message of failure or the activation overtime in the GGSN, the GGSN can return to the IP Multicast Access (which is defined in 3GPP TS29.061).
208 If necessary, encryption process is performed between the UE and the SGSN.
209 the SGSN sends a “MBMS Context Establishment Request” message to the GGSN, which contains IP multicast address and APN.
210 the GGSN performs the signaling interaction with the BM-SC to authenticate the MBMS service and the UE after receiving the message in step 209.
211a If the GGSN has not established the bearer context for this MBMS service yet, it sends a “Bearer Request” message to the BM-SC, which contains IP multicast address and the APN.                211b The BM-SC sends a “Bearer Response” message to the GGSN, which contains bearer context information of this MBMS service. The BM-SC adds the identifier of this GGSN into the downlink node list of the BM-SC bearer context.        If this MBMS service has not yet been allocated with a Temporary Mobile Group Identifier (hereinafter referred to as TMGI), the BM-SC allocates a new one and transfers it to the GGSN and SGSN by the message of “Bearer Response”, and to the UE by a message of “Activate MBMS Context Accept” (215).        
If the state of the bearer context is activating, the BM-SC initiates the session start process with the GGSN.
212 the GGSN generates the MBMS UE context and sends a “MBMS Context Generation Response” message to the SGSN.
213a If the SGSN has not yet established the bearer context for this MBMS service, it sends the “Bearer Request” message to the GGSN, which contains the IP multicast address and the APN.
213b The BM-SC sends the “Bearer Response” message to the GGSN, which contains the MBMS bearer context information. The BM-SC adds the identifier of this GGSN into the downlink node list of the BM-SC bearer context.
If the state of the bearer context is activating, the BM-SC initiates the session start process with the GGSN.
214 if at least one RAB of PS domain has been established for this UE, the SGSN transfers the MBMS UE context information to the RAN.
215 the SGSN sends the “Activate MBMS Context Accept” message to the UE. The SGSN does not need to wait till the complete of step 212 to send this message.
In practical networks, it is possible that new equipments with MBMS capability coexist with the remnant ones that are in service currently. For instance, the SGSN that is capable of the MBMS can connect with the old version RNC. To guarantee the backward compatibility of the MBMS service and the interworking among the equipments with different versions, all old version equipments should provide the UE with the MBMS service through the mode of Re199.
But for the MBMS service, to activate the MBMS UE context when joining in the MBMS service, the UE discriminates the MBMS contexts and notifies the SGSN with the MBMS service identifier instead of selecting the NSAPI for the MBMS. Therefore, the SGSN has no NSAPI to utilize when establishing 1u user interface for UE with the process of Re199, and as there is no correspondence relation between the NSAPI and the RAB identifier, the UE has no idea in discriminating the services corresponding to the data from the bearers. The SGSN can not replace the RAB identifier directly with the MBMS service identifier, for a RAB identifier is 8-bit long, while an MBMS service identifier, such as TMGI, is 32-bit long.